It’s made clear that Mayu doesn’t ever want to be parted from her twin sister, and will go to any lengths to ensure that they stay together. A fair bit of the things she says and does are due to Sae manipulating and possessing her, but I don’t think that begins until after you defeat Miyako, and that voiceover happens before you ever set foot in All Gods’ Village. If you look at the translation of the ending song “Chou”, you get a little more insight into her thoughts and character. She makes it clear from the very beginning that she doesn’t want to ever let Mio go (the voiceover we hear before the game starts is Mayu’s voice), and in the Hellish Abyss ending when Mio has been stricken blind by the Abyss, Mayu seems happy that her sister now has to rely on her, and thus can never leave her. The way I see it, Mayu is less of a victim and more of a person getting exactly what she wants.
